Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $205.26   6.648%   $218.9057   $218.91   $218.90   $218.90  
  Purchase #2 $13.44   11.365%   $14.9675   $14.97   $14.95   $15.00  
  Purchase #3 $124.46   3.192%   $128.4328   $128.43   $128.45   $128.40  
  Purchase #4 $247.74   13.651%   $281.559   $281.56   $281.55   $281.60  
  Purchase #5 $180.27   4.211%   $187.8612   $187.86   $187.85   $187.90  
  Purchase #6 $69.60   6.622%   $74.2089   $74.21   $74.20   $74.20  
  Purchase #7 $113.48   3.142%   $117.0455   $117.05   $117.05   $117.00  
  7 purchase totals = $954.25       $1,022.9806   $1,022.99   $1,022.95   $1,023.00